The potential distribution of the harvestman Geraeocormobius sylvarum Holmberg 1887 (Opiliones, Gonyleptidae, Gonyleptinae) from Argentina, Brazil and Paraguay, is modeled using the presence-only, GIS-based method Bioclim. The model was run on 2.5 min resolution climate layers using 19 derived bioclimatic variables. The bioclimatic profile of the species is described, and presumable limiting factors in each part of the range are discussed. Modeled distribution of G. sylvarum shows a remarkable correspondence to the Alto Paraná Atlantic forest ecoregion, with a marginal presence around the Araucaria forests and in gallery/flood forests towards the Southwest. Results support the 650 km yungas-Mesopotamia disjunction, as previously proposed, and reveal that localities in northwestern Argentina have extreme values concerning seasonality parameters with remarkably decreased rainfall in winter. Evidence suggesting that the disjunct pattern may have been derived by antropic introduction is briefly discussed.